Breast cancer is a common and, often fatal disease. Estrogen frequently plays an important role in sustaining its growth and therapies directed at interruption of estrogen action are useful in treating many patients. However, many breast cancers which are initially estrogen-responsive can evolve to an estrogen-unresponsive state. A major clinical problem is that chemotherapy for the treatment of estrogen-independent metastatic breast cancer does not effectively increase survival. Thus the long-term goal of the proposed studies is to attempt to potentiate the cytotoxic effects of available chemotherapeutic agents by other means. Cell death can occur via two pathways, necrotic cell death or programmed cell death. The latter is a physiological process whereby the cell is activated by specific signals to undergo an active cascade of events which results in its death. Preliminary work has shown that estrogen-responsive human breast cancer cells can undergo programmed cell death after estrogen withdrawal. Further, this pathway can be induced by fluoropyrimidines in estrogen-unresponsive human breast cancer cells although it is no longer triggered by hormone withdrawal. The studies proposed here will seek to further our understanding of the biochemical and morphological events which comprise the programmed cell death pathway in human breast cancer cells. An initial goal of this work will be to determine the dose-response relationship for the induction of programmed cell death in hormone-independent human breast cancer cells by selected chemotherapeutic agents. These models can then be used to address the potential roles of intracellular Ca++, members of the transforming growth factor beta family, and protein kinase C activity as mediators of this death process in hormone-independent breast cancer. Since growth of a cancer is determined by the relationship between the rate of cell proliferation and rate of cell death, a possible treatment strategy would be to attempt to develop methods of increasing chemotherapy-induced cell death by modulating these variables. In order to have any realistic chance of success much additional information must be obtained about the fundamental mechanisms of programmed cell death in malignant breast cells through the studies proposed here.